Final answer:
The first-line imaging for a woman under 30 years old with a breast mass is typically a mammogram, although the contrast may be more difficult to observe. An MRI can be used as a supplement to improve detection and eliminate false positives.
Step-by-step explanation:
The first-line imaging for a woman under 30 years old with a breast mass is typically a mammogram. Although mammograms are generally recommended for women aged 50 and older, they can still be used for younger women to detect any lumps or regions of increased density within the breast. However, the contrast may be more difficult to observe in younger women with denser breasts. If necessary, a MRI (Magnetic resonance imaging) can be used as a supplement to mammograms to improve detection and eliminate false positives.